The invention relates more particularly to an apparatus for assisting with the ventilation of a patient who is breathing with successive cycles, each of which includes an inhalation phase and an exhalation phase, of the type especially including an inhalation assistance mode during which the apparatus delivers to the patient, during the inhalation phase triggered by the beginning of inhalation of the patient, a flowrate of gas under a pressure equal to a given reference value and which includes:
a source of pressurized gas, an outlet orifice of which delivers a flowrate of pressurized gas intended to be transmitted to the upper airways of the patient;
a transmission circuit including a main inhalation conduit connected to the outlet orifice of the source of pressurized gas and to a mask, especially of the face mask type, intended to be worn by the patient, and including a secondary exhalation conduit connected to the mask and to a discharge orifice;
a flow regulating valve which is interposed in the main conduit and which is operated by an operating circuit of the apparatus on the basis especially of the values of the flowrate and of the pressure of the gas in the main inhalation conduit measured downstream of the regulating valve, especially in order to determine the beginning of the inhalation phase; and
an exhalation valve interposed in the secondary exhalation conduit which is made to open by the operating circuit of the apparatus, especially during the exhalation phase of each breathing cycle of the patient in respiratory aid mode.
An example of such a type of apparatus is illustrated in the documents WO-A-89 10768 and EP-A-0,425,092.
When it is used in inhalation assistance mode, all of the flowrate of pressurized gas, especially pressurized air, is transmitted to the mask (worn by the patient) during each inhalation phase, the exhalation valve being closed.
During each exhalation phase, the exhalation valve is open and the inlet orifice of the mask to which the main conduit is connected is thus vented to ambient air so that the patient can freely exhale the air inhaled during the previous inhalation phase.
During this exhalation phase, the function of the flow regulating valve is to interrupt the flowrate in the main inhalation conduit by bypassing, for example, the flow of pressurized air produced by the source to outside the apparatus.
The flow regulating valve must thus allow a relative pressure at the inlet of the mask, during the exhalation phase, which is predetermined and may be close to zero.
The operating circuit detects the exhalation phase of the patient when the pressure measured in the main conduit is greater than a reference value or when the flowrate of this conduit drops below a reference threshold value.
A classic design of the regulating valve described in document WO-A-89 10768 is such that operation of the valve of "all-or-nothing" type involves the use of an axial compressor as a source of pressurized air.
In this document, the variation in air pressure is here effectively obtained by causing the rotational speed of the compressor to vary, which requires several respiratory cycles to establish a determined value for the pressure.
What is more, that gives rise to noises of acceleration and deceleration of the motor of the compressor which are particularly troublesome in the case of a home respiratory-aid apparatus.
In document EP-A-0,425,092, the ventilator described and represented is connected to a mask for supplying pressurized air which is equipped with ventilation openings which allow both the exhalation and the passage of a residual flowrate into the supply circuit.
These openings, which may or may not be equipped with a check valve, must be sufficiently large to allow the exhaled gases to be discharged, even under a low exhalation pressure.
In contrast, their size is limited by the need to have a reduced leakage flowrate, which necessarily means that exhalation cannot take place at a very low pressure.
The application of the principle described in this document implemented commercially by the RESPIRONICS company is known under the trade name of BIBAP Ventilatory Support System.
This apparatus does not make it possible to have an exhalation pressure (EPAP) less than 4 mbar, which may constitute a hindrance for the patient.
The object of the invention is to provide an apparatus for aiding the ventilation of the type mentioned previously which overcomes the drawbacks which have just been outlined.